My accepted C# O(n) Solution

Use Stack to store all the numbers, use switch to operate the numbers.

here's the code:
public class Solution {
public int EvalRPN(string[] tokens) {
bool isNum;
Stack<int> numStack= new Stack<int>();// store the numbers in a stack
int OpdCount=0;
String temp="";
for(int i=0; i< tokens.Length; i++)
{
int num;
isNum=int.TryParse(tokens[i], out num);//check if it is a number
if(isNum)// if it is a number, push it into the stack
{
numStack.Push(num);
temp +=tokens[i];
}
else // if it's not a number, then pop out the last 2 numbers in stack and operater
{
OpdCount++;
int top1= numStack.Pop();
int top2= numStack.Pop();
numStack.Push(operateNums(top2, top1, tokens[i]));
}